Kotlin ActivityResultContracts TakePicture 结果总是返回 false

您的代码的问题是，您将empty Uri传递给launch，而相机应用程序无法保存这Uri中的图像.如果打开TakePicture类或将鼠标光标放在其上，您将看到以下信息:

一个ActivityResultContract来拍摄照片，并将其保存到 提供的内容-URI.如果图像保存到 给定的URI.

换句话说，TakePicture类不会自动为您创建File，您将不得不自己创建File并提供Uri.

我将假设一个简单的场景，即你想要在应用程序中为一些临时任务拍照.要实现此目标，您需要了解代码中缺少的一些步骤:

1. 由于您需要创建File并将其公开给相机应用程序，因此您需要使用File Provider创建规则并在Manifest文件中声明它.
2. 创建File并将其Uri与FileProvider一起公开的函数.

让我们从file_paths.xml开始(res/xml内部):

<?xml version="1.0" encoding="utf-8"?>
<paths>
    <cache-path
        name="cache_pictures"
        path="/" />

</paths>

出于暂时保存文件的 idea ，我在这里使用了cache-path.

在Manifest文件中:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">

    <uses-permission android:name="android.permission.CAMERA" />

    <application ...>

        <activity .../>

        <provider
            android:name="androidx.core.content.FileProvider"
            android:authorities="${applicationId}.provider"
            android:exported="false"
            android:grantUriPermissions="true">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/file_paths" />
        </provider>

    </application>

</manifest>

扩展以创建File并返回Uri:

fun Context.createTempPictureUri(
    provider: String = "${BuildConfig.APPLICATION_ID}.provider",
    fileName: String = "picture_${System.currentTimeMillis()}",
    fileExtension: String = ".png"
): Uri {
    val tempFile = File.createTempFile(
        fileName, fileExtension, cacheDir
    ).apply {
        createNewFile()
    }

    return FileProvider.getUriForFile(applicationContext, provider, tempFile)
}

在本例中，缓存文件夹使用的是cacheDir.如果在这里改到filesDir，一定要在cache-path号公路上从file_paths.xml改到files-path.

现在，在Composable Screen分钟内，你可以得到这样的东西:

@OptIn(ExperimentalPermissionsApi::class)
@Composable
fun SomeScreen() {
    val context = LocalContext.current
    var currentPhotoUri by remember { mutableStateOf(value = Uri.EMPTY) }
    var tempPhotoUri by remember { mutableStateOf(value = Uri.EMPTY) }

    val cameraLauncher = rememberLauncherForActivityResult(
        contract = ActivityResultContracts.TakePicture(),
        onResult = { success ->
            if (success) currentPhotoUri = tempPhotoUri
        }
    )

    val cameraPermissionState = rememberPermissionState(
        permission = Manifest.permission.CAMERA,
        onPermissionResult = { granted ->
            if (granted) {
                tempPhotoUri = context.createTempPictureUri()
                cameraLauncher.launch(tempPhotoUri)
            } else print("camera permission is denied")
        }
    )

    Column(
        modifier = Modifier.fillMaxSize(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        AnimatedVisibility(visible = currentPhotoUri.toString().isNotEmpty()) {
            // from coil library 
            AsyncImage(
                modifier = Modifier.size(size = 240.dp),
                model = currentPhotoUri,
                contentDescription = null
            )
        }

        Button(onClick = cameraPermissionState::launchPermissionRequest) {
            Text(text = "Take a photo with Camera")
        }
    }
}